1. Field of the Invention
The present invention relates in general to fuel sensors, and more particularly to fuel sensors of a type which senses the mixing ratio of methanol in a mixture of gasoline and methanol.
2. Description of the Prior Art
For dealing with the energy crisis and air polution problem, it has been proposed to use as the fuel of an automotive internal combustion engine a mixed fuel, such as a mixture of gasoline and methanol. Japanese Patent First Provisional Publication 56-138431 shows one of the engine systems of such type, in which a fuel injection system is used. In the engine system of this publication, a measure is employed in which for practically operating the engine on the methanol-mixed gasoline, the fuel injection amount and ignition timing are controlled in accordance with the mixing ratio of methanol in the mixed fuel, which ratio is measured by a fuel sensor disposed in a fuel container.
The fuel sensor is of a capacitance type which comprises a pair of spaced electrode plates submerged in the fuel. The dielectric constant of the fuel is detected by measuring the capacitance established between the electrode plates, and the mixing ratio of methanol is derived from the dielectric constant.
In operation, the signal representative of the capacitance established between the electrode plates is transmitted through elongate lead wires to a separate control circuit where subsequent signal conversion and processing are carried out.
However, usage of such lead wires tends to induce noise on the signal. That is, since the capacitance practically established between the electrode plates is quite small (that is, of the order of 10.sup.-12 F), the signal representative of the capacitance is greatly affected by noises and floating capacitance thereby causing a considerable error in measuring the mixing ratio of methanol in the fuel.